1. Field of the Invention
The present invention relates to free-stream wind and water mills, more particularly to multi-turn helicoid structures set in rotary motion by the wind or by free-flowing water, thereby extracting kinetic energy from the flow.
2. Description of the Prior Art
The construction and use of wind and water mills dates back to pre-history. Nevertheless, renewed interest has arisen even to the extent that recent surveys of wind mill technology have been funded by U.S. Government Agencies. A review of the traditional as well as the newer technology has been prepared in a report by the Mitre Corporation, written by Frank A. Eldridge, under Grant No. AER-75-12937 of the National Science Foundation, dated October 1975, entitled "Wind Machines" (Superintendant of Documents, U.S. Printing Office, Stock No. 038-000-00272-4; re-issued as book with the same title by Van Nostrand Reinhold Co. New York, 2nd Edition 1980). This report also contains a glossary of terms to which I will adhere in the following.
Water mills, on the other hand, have had a continuous evolutionary development within the progress of modern technology, which led ultimately to the various turbine designs of our present hydro-electric power stations.
My invention is best described and understood with reference to wind mills. Just as all wind mills do, the device I have invented "extracts" or "converts" some of the kinetic energy of the moving fluid into mechanical or heat energy or electrical energy for useful purposes.
Wind mills are broadly classified into two types: drag types and lift types. The lift types are using blades which look much like aircraft wings or propellers. (Propellers are sometimes called "air screws" or "helical blades", particularly in the British literature; but these have nothing in common with the structure of my present invention.) These lift type blades require intricate aerodynamic design, must run at high speed to be efficient and must be very large, because a single one of them does not intercept a large cross sectional area of the wind flow. Large speed and large size are requirements which are difficult to reconcile.
The drag type of wind mills, such as the Savonius-Rotor, are simple in construction but they depend on a difference in drag exerted by the wind on each of the rotor wings or "shovels", this difference being caused solely by the different geometrical shapes presented to the wind, namely, a convex surface in one view, a concave "bucket" in the other view. Such rotors are therefore inherently inefficient and they experience forces that are not utilized to generate rotational momentum, yet these extra forces must also be sustained by the structures. The above factors impose severe limits on the size of the designs, on the drag type as well as on the lift type of devices.
Propeller type designs are also limited by gyroscopic forces which appear when their axis changes its orientation.
It is the objective of my invention to build a wind energy converter, and, more broadly, an energy converter for the kinetic energy of any free-stream fluid flow, that is of simple shape, that presents essentially no spurious surface area to the flow which could produce a counter-momentum to the desired rotational momentum, and that permits to intercept with its working-surface a large cross sectional area of the flow. It is a further objective to control and balance gyroscopic forces within the system itself, so that no such forces interfere with the free and easy adjustment of the total system to the flow vector or to changes in the flows direction.
Surprising as this may seem, the structure which can accomplish these feats is a helicoid, no other than a large Archimedian screw or Archimedian spiral. To be precise, there is a difference between a spiral, a screw, a helix, and a helicoid. The latter is defined in the Random House dictionary as follows: "A warped surface generated by a straight line moving so as to cut or touch a fixed helix." A helix is, of course, a line on a cylinder progressing with each full revolution a certain distance called the pitch towards the right or the left (assuming the cylinder lies horizontal).
To utilize helicoids as fluid flow extractors, it is only necessary to incline them slightly to the direction of the flow, the angle of inclination being in the order of the mean pitch angle of the helicoid or slightly larger. A duct is not necessary. The prior use of the Archimedian screw was generally that of a driven device, e.g. for raising water. In modern screw compressors and pumps it is used in essentially the same way. In each of these cases it is enclosed in a duct, and the fluid is transported parallel to its axis. No wind mill of the helicoid type is found among the various types discussed by Eldridge (quoted above).
There is one wind mill type used in the Phillipines which uses cloth sails on a wooden frame, the sail sections forming a single turn of a screw (instead of being broken-up into separate blades and mounted in one plane, as usual). It is therefore also called "helical sail wind mill" or "cyclo wind mill". Its axis is pointed into the wind, therefore more than one circle of sails would be useless. It is a large, slow-speed device, designed for pumping water. (A description can be found in the VITA-Publication "Helical Sail Windmill", ISBN 0-86619-036-8, 1980).
There is yet another type of wind mill which uses two twisted surfaces that form two, unconnected, helical rotors at some distance from a thin central shaft. It resembles a Savonious Rotor which is split and twisted. In some models the central shaft may even be missing, while sails of cloth are streched out by ropes their spacing being secured by short sticks of wood or metal; these are known as tetra-helix sails. In all cases, the pitch of these helical surfaces is rather steep, with a pitch angle near 80 degrees, and they make just one turn over the full length of the structure, which length is many times greater than the diameter and spacing of the surfaces. This type of helical double rotor, as well as the tetra-helix sails, are described, yet only scantilly, in the book by Dermot McGuigan entitled "Harnessing the Wind for Home Energy", Garden Way Publishing, Charlott, Vt. 05445.
In the just mentioned designs there is no central cylindrical body which holds the helical surfaces, as in my present invention, and the two helical sails do not form one continuous surface, as do the helical sheets in my present invention. These designs lack completely the strength and stability of the helicoid structure of the present invention. It is even mentioned as an advantage that the cloth sails will collapse in case of excessive winds. My present invention is designed to withstand the strongest of winds.
With respect to the commercial importance of any improved wind mill design, it can be pointed out that wind blows everywhere, whereas wave or tidal power are only awailable at certain coast lines. It has therefore been estimated that in due course wind power may generate over 10.sup.13 kWh/year, whereas wave and tidal power may perhaps achieve 1/10th of this level in the United States. (See the book by Donald E. Carr: "Energy and the Earth Machine", Norton, N.Y., 1976 Chapter 9, pp. 115-123).